Gelled hydrocarbons for oilfield processes, phosphate ester compounds useful in gellation of hydrocarbons and methods for production and use thereof

ABSTRACT

Phosphate esters useful for gelling hydrocarbons in combination with a metal source are disclosed along with methods of preparation of the phosphate esters. Fouling in oil refinery towers has been attributed due to distillation of impurities present in phosphate esters used to gel hydrocarbons for oil well fracturing. The improved method of preparation of the phosphate ester results in a product that substantially reduces or eliminates volatile phosphorus, which is phosphorus impurities that distill up to 250° C., and increases the high temperature viscosity of the hydrocarbon gels formed using the phosphate esters.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional application of U.S. application Ser. No. 12/367,841filed Feb. 9, 2009, which is presently pending. U.S. application Ser.No. 12/367,841 and the present application claim priority under 35U.S.C. §119(e) to U.S. provisional patent application No. 61/027,342filed Feb. 8, 2008 and to U.S. provisional patent application No.61/030,040 filed Feb. 20, 2008.

FIELD

The invention relates to gelled hydrocarbons for oilfield processes,phosphate ester compounds useful in gellation of hydrocarbons andmethods for production and use thereof. Phosphate esters of greatestinterest herein are interchangeably referred to as dialkyl phosphates,phosphate diesters and dialkyl phosphate esters.

BACKGROUND

It has been long known that certain phosphate esters are useful ingenerating gelled liquids, particularly gelled hydrocarbons. Gelledhydrocarbons, due to their high viscosity and ability to suspend solids,have found several applications in the field of oil recovery. Morecommonly they are used in stimulation related processes.

Several patents have been issued based on this concept, see for exampleU.S. Pat. Nos. 4,153,649, 4,622,155, 5,057,233, 5,190,675, and6,261,998.

Despite the effectiveness of gelled liquid hydrocarbons in formingfractures in subterranean formations, one particular problem with theiruse has been described in literature. It has been reported thatrefineries processing oil produced from formations fractured with gelledliquid hydrocarbons have experienced fouling of the distillation towers.Analysis of the fouling material has revealed a high phosphorus contentwhich has been postulated to originate from a phosphate ester distillingat 230-290° C. In response, several patents have been issued forformulations that are claimed to have low volatile phosphoruscontribution in the distillate, see U.S. Pat. Nos. 7,066,262, 6,511,944,and US Applications 20070032387 and 20070173413. It has been speculatedin the patent literature that decomposition of phosphate esters to lowermolecular weight phosphorus compounds and/or the presence of certain lowboiling impurities in the commercial mixture are the sources of volatilephosphorus species. Therefore, the use of less volatile trialkylphosphate in the manufacturing process of the phosphorus based gellingagent as well as the replacement of phosphate esters withmonoalkanephosphonic acid monoesters has been suggested as a method toameliorate the fouling.

A number of studies on the pyrolysis and combustion of phosphate estersand alkylphosphonate esters have been published, see for example, H. E.Baumgarten, R. A. Setterquist J. Am. Chem. Soc. 1957, 79, 2605-2608 andP. A. Glaude, H. J. Curran, W. J. Pitz, C. K. Westbrook Kinetic Study ofthe Combustion of Phosphorus Containing Species, Article presented at1999 Fall Meeting of the Western State Section of the CombustionInstitute, Irvine, Calif., Oct. 25-26, 1999, and the references therein.

At elevated temperatures and in the presence of oxygen both phosphatesand phosphonates decompose to P₂O₅ (solid) along with CO, CO₂, H₂O, Cand CH₄. In the absence of oxygen, P₂O₅ along with olefins are produced.P₂O₅ is the product of dehydration of H₃PO₄, neither of which can bedistilled under atmospheric pressure (Erwin Riedel, Anorganishe Chemie,5th edition, 2002, pp. 492-495 and Ch. E. Housecroft, A. G. Sharpe,Inorganic Chemistry first edition, 2001, pp 341-342).

SUMMARY

In accordance with a broad aspect of the present invention, there isprovided a method for producing an asymmetric dialkyl phosphate ester,the method comprising: (a) reacting a precursor dialkyl phosphite with atransesterifying agent to obtain a transesterified asymmetric dialkylphosphite; (b) removing any unreacted transesterifying agent and anyunreacted precursor dialkyl phosphite from the transesterifiedasymmetric dialkyl phosphite; and (c) reacting the transesterifiedasymmetric dialkyl phosphite to form an asymmetric dialkyl phosphateester.

In accordance with another broad aspect of the present invention, thereis provided an asymmetric dialkyl phosphate ester according to theformula:

R¹ is a straight chained alkyl group having 1 to 4 carbon atoms; and R²is alkyl group having 6 to 20 carbon atoms or an alkoxyalkyl group,C_(n)H_((2n+1))O(CH₂CH₂O)_(x)CH₂CH₂—, where n is about 6 to 16 and x isabout 1 or 2, and wherein any amount of phosphate triester that distilswithout decomposition at temperatures up to 250° C. (ASTM D86) ismaintained below 1% by weight.

In accordance with another broad aspect of the present invention, thereis provided a gelled hydrocarbon liquid comprising: a hydrocarbonliquid; 1 to 15 kg/m³ (w/v hydrocarbon liquid) of a gelling agentincluding an asymmetric dialkyl phosphate ester, wherein any amount ofphosphate triester that distils without decomposition at temperatures upto 250° C. is maintained below 1% by weight in the gelling agent, and0.1 to 7.5 kg/m³ (w/v hydrocarbon liquid) of a polyvalent metal crosslinking agent.

In accordance with another broad aspect of the present invention, thereis provided a method of treating a subterranean well comprising:providing a hydrocarbon liquid, gelling the hydrocarbon liquid to obtaina gelled hydrocarbon liquid by adding (i) a gelling agent including anasymmetric dialkyl phosphate ester; and (ii) a polyvalent metal crosslinking agent, wherein any amount of phosphate triester that distilswithout decomposition at temperatures up to 250° C. is limited to lessthan or equal to 10 ppm phosphorus in the gelled hydrocarbon distillate;introducing the gelled hydrocarbon liquid to a subterranean well; andmanipulating the gelled hydrocarbon liquid to treat a formation accessedby the subterranean well.

It is to be understood that other aspects of the present invention willbecome readily apparent to those skilled in the art from the followingdetailed description, wherein various embodiments of the invention areshown and described by way of illustration. As will be realized, theinvention is capable for other and different embodiments and its severaldetails are capable of modification in various other respects, allwithout departing from the spirit and scope of the present invention.Accordingly the detailed description and examples are to be regarded asillustrative in nature and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the drawings, several aspects of the present invention areillustrated by way of example, and not by way of limitation, in detailin the figures, wherein:

FIG. 1 is a ³¹P NMR spectrum of the commercially available phosphorousester mixture, Rhodafac LO-11A-LA, used in oil based gels (δ −0.63 ppm(C₂H₅O)₃P(O) (0.8%), −0.52 ppm (R²O)(C₂H₅O)₂P(O) (4.2%), −0.42 ppm(R²O)₂(C₂H₅O)P(O) (4.3%), −0.32 (R²O)₃P(O) (0.6%), 1.47 ppm(C₂H₅O)₂P(O)(OH) (16%), 1.67 ppm (R²O)(C₂H₅O)P(O)(OH) (52.7%), 1.71 ppm(R²O)₂P(O)(OH) (11.2%), 2.81 ppm (C₂H₅O)P(O)(OH)₂ (6.3%), 2.94 ppm(R²O)P(O)(OH)₂ 3.9%), where R²=C₈H₁₇ and C₁₀H₂₁).

FIG. 2 is a ¹H NMR spectrum of ethyloctylphosphite.

FIG. 3 is a ³¹P NMR spectrum of ethyloctylphosphite.

FIG. 4 is a ¹H NMR spectrum of ethyloctylphosphate.

FIG. 5 is a ³¹P NMR spectrum of ethyloctylphosphate.

FIG. 6 is a plot of the viscosities of the gels obtained using RhodafacLO-11A-LA and pure ethyloctylphosphate at various temperatures.

FIG. 7 is a plot of the viscosities of the gels obtained withethyloctylphosphate and mixture of ethyloctylphosphate and diethylphosphate.

FIG. 8 is a plot of the viscosities of the gels obtained withethyloctylphosphate and mixture of ethyloctylphosphate anddioctylphosphate.

FIG. 9 is a plot of the viscosities of the gels obtained withethyloctylphosphate and mixture of ethyloctylphosphate anddiethyloctylphosphate.

FIG. 10 is a plot of the viscosities of the gels obtained withethyloctylphosphate and mixtures of ethyloctylphosphate andoctylphosphate at two ratios.

FIG. 11 is a plot of the viscosities of the gels obtained with productsfrom Examples 18 to 22.

FIG. 12 is a plot of the viscosities of the gels obtained with productsfrom Examples 23, 24 and 26.

FIG. 13 is a plot of the high temperature viscosities of the gelsobtained with Rhodafac LO-11A-LA and products from Examples 1, 18 and20.

DESCRIPTION OF VARIOUS EMBODIMENTS

The description that follows and the embodiments described therein, areprovided by way of illustration of an example, or examples, ofparticular embodiments of the principles of various aspects of thepresent invention. These examples are provided for the purposes ofexplanation, and not of limitation, of those principles and of theinvention in its various aspects.

Since the net decomposition product of phosphate esters is phosphoricacid or P₂O₅ (solid), we hypothesized that the presence in thecommercial phosphate ester mixture of certain phosphorus species thatcan be distilled without decomposition at temperatures below 250° C.using ASTM D86, or at most below 300° C., are in fact the source of mostdistillable phosphorus.

A typical phosphate ester mixture used in gelling liquid hydrocarbons isproduced commercially by reacting triethylphosphate and P₂O₅ at elevatedtemperatures. This reaction produces a polyphosphate intermediate whichis further reacted at elevated temperatures with an alcohol (typically acommercial mixture of n-C₈ and n-C₁₀ alcohols) to produce a mixture ofmono, di and trialkyl phosphate esters. The exact composition of themixture of products and the relative ratios of the components has notbeen reported in literature. We hypothesized that besides the desiredasymmetric dialkylphosphate some symmetric dialkylphosphate,monoalkylphosphate and trialkylphosphate species might form according toEquation 1.

To study the matter, we acquired NMR spectra for a commerciallyavailable gelling agent and some of the components we hypothesized to bepresent. Indeed, the ³¹ P{¹H} NMR spectrum of a mixed phosphate estergelling agent, Rhodafac LO-11A-LA, prepared by the reaction described inEquation 1 shows multiple singlet signals which confirms the complexnature of the product mixture (FIG. 1). We independently prepared eachassumed component of the mixture and performed NMR measurements onspiked samples. In this manner, we have been able to ascertain thatRhodafac LO-11A-LA consists more or less of 0.8% (C₂H₅O)₃P(O), 4.2%(R²O)(C₂H₅O)₂P(O), 4.3% (R²O)₂(C₂H₅O)P(O), 0.6% (R²O)₃P(O), 16%(C₂H₅O)₂P(O)(OH), 52.7% (R²O)(C₂H₅O)P(O)(OH), 11.2% (R²O)₂P(O)(OH), 6.3%(C₂H₅O)P(O)(OH)₂, and 3.9% (R²O)P(O)(OH)₂, where R²=n-C₈H₁₇ or n-C₁₀H₂₁.Each pair of octyl and decyl substituted compounds yield isochronous NMRsignals and cannot be distinguished.

Since the phosphate triesters are the species believed to contribute tothe majority of distillable phosphorus, a synthetic route to producingan asymmetric dialkyl phosphate ester without coincidental production ofthe corresponding triesters is desirable.

The present invention provides gelled hydrocarbons, liquid hydrocarbongelling agents and methods of producing and using liquid hydrocarbongelling agents and gelled hydrocarbons which may reduce or eliminatetheir amount of distillable phosphorus content and may increase the hightemperature viscosity of the hydrocarbon gels formed.

Thus, in one embodiment, the invention provides an asymmetric dialkylphosphate ester, which contains, if any, less than 1% by weight of aphosphate triester that distils without decomposition at temperatures upto 250° C. (using the standard ASTM D86). The asymmetric dialkylphosphate may be represented according to formula [i], which is:

wherein,

-   -   R¹ is a straight chained alkyl group having 1 to 4 carbon atoms;        and    -   R² is an alkyl group having 6 to 20 carbon atoms or an        alkoxyalkyl group, C_(n)H_((2n+1))O(CH₂CH₂O)_(x)CH₂CH₂—, where n        is between about 6 and 16 (inclusive) and x is 1 or 2.

In the definition of R¹ and R², the difference between the two chainlengths should be greater than or equal to about 4 atoms. For example,if R¹ and R² are each alkyl chains, the difference between them is atleast 4 atoms. The same is true if R² is an alkoxyalkyl group.

In another embodiment, the invention incorporates a composition ofmatter that along with the asymmetrical dialkyl phosphate ester includesa monoalkyl phosphate ester and/or a symmetrical dialkyl phosphateester.

The useful symmetrical dialkyl phosphate ester is a higher molecularweight phosphate diester, for example, having ester substituents greaterthan C5. These phosphorus compounds tend not to distill withoutdecomposition at temperatures up to 250° C. For example, in oneembodiment, a mixture of asymmetric and symmetric dialkyl phosphates mayinclude species of the formulae:

wherein

-   -   R¹ is a straight chained alkyl group having 1 to 4 carbon atoms;        and    -   R² is alkyl group having 6 to 20 carbon atoms or an alkoxyalkyl        group, C_(n)H_((2n+1))O(CH₂CH₂O)_(x)CH₂CH₂—, where n is about 6        to 16 and x is about 1 or 2.

In the definition of formula [i], the difference between the two chainlengths, R¹ and R², should be greater than or equal to about 4 atoms.

The asymmetric dialkyl phosphate, or the mixture of asymmetric andsymmetric dialkyl phosphates, may contain an amount of a monoalkylphosphate ester. As such a mixture may also be provided includingcompounds corresponding to the formulae:

wherein

-   -   R¹ is a straight chained alkyl group having 1 to 4 carbon atoms;    -   R² is alkyl group having 6 to 20 carbon atoms or an alkoxyalkyl        group, C_(n)H_((2n+1))O(CH₂CH₂O)_(x)CH₂CH₂—, where n is about 6        to 16 and x is about 1 or 2; and    -   R³ is a straight chain alkyl having 5 to 20 carbon atoms or        alkoxyalkyl C_(n)H_((2n+1))O(CH₂CH₂O)_(x)CH₂CH₂—, where n is        about 6 to 16 and x is about 1 or 2.

In the definition of formula [i], the difference between the two chainlengths, R¹ and R², should be greater than or equal to about 4 atoms.

It is to be understood that a composition of interest includes theasymmetric dialkyl phosphate ester with or without a monoalkyl phosphateester and/or a dialkyl phosphate ester. Such compositions may be usefulas gelling agents for hydrocarbon liquids. In a gelling agent, it may beuseful to control the relative concentrations of the species in order toselect for properties such as, for example, cost and gelling propertiessuch as any or all of gelling speed, gel viscosity, gelling duration,resistance to thermal breaking, etc. For example, in one embodiment, acomposition may include 50-100% by weight of an asymmetric dialkylphosphate ester of the formula [i]; 0-50% by weight of a symmetricdialkyl phosphate ester of the formula [ii]; and 0-10% by weight of amonoalkyl phosphate ester of the formula [iii].

In any such gelling agent composition, impurities such as phosphatetriesters that distill without decomposition up to 250° C., and possiblyup to 300° C., should be limited to less than 1% by weight. If possible,the gelling agent should contain no more than trace amounts, and in oneembodiment substantially no, phosphate triesters. If possible, allvolatile phosphorus, which generally includes phosphorus impurities thatdistill without decomposition up to 250° C. or up to 300° C., should bemaintained below 1% by weight and if possible kept to no more than traceamounts. Such phosphorus impurities may include, for example, lowmolecular weight diesters and monoesters, such as phosphate diesters andmonoesters with ester substituents having less than five carbon atoms.

As detailed hereinafter, a method is proposed for obtaining reactionproducts including an asymmetric phosphate diester and, possibly,varying amounts of symmetric phosphate diesters and/or phosphatemonoesters substantially without the above-noted impurities. Reactionproducts having different selected gelling properties can be prepared byreacting specific reactants and/or by varying the molar ratios in whichthe reactants are reacted.

These phosphate esters may be prepared starting from a dialkyl phosphiteand a transesterifying agent. In particular, in one embodiment, a methodmay include (a) reacting a precursor dialkyl phosphite with atransesterifying agent to obtain a transesterified asymmetric dialkylphosphite; (b) removing transesterifying agent and any unreactedprecursor dialkyl phosphite from the transesterified asymmetric dialkylphosphite; and (c) reacting the transesterified asymmetric dialkylphosphite to form an asymmetric dialkyl phosphate ester.

The precursor dialkyl phosphite may be symmetric or asymmetric. Sincetransesterification becomes more difficult with longer chain moieties,the precursor dialkyl phosphite may be low molecular weight, withstraight chain C1 to C4 alkyl groups. In one embodiment, the dialkylphosphite precursor may be selected from dimethyl, diethyl, dipropyl ordibutyl phosphites. A more readily available phosphite, is diethylphosphite.

In the reaction, the transesterifying agent acts to replace one or bothof the alkyl groups of the dialkyl phosphite precursor. As such, auseful transesterifying agent has a carbon chain backbone that isdissimilar to the alkyl groups of the phosphite precursor. Generally, atransesterifying agent may include a straight chain moiety with aterminal hydroxyl or thiol group. Reasonably, transesterifying agentswith terminal hydroxyl groups are most useful. For example, alcoholssuch as primary alcohol or an ether of an alcohol are useful. In oneembodiment, for example, a primary alcohol with a chain length including6 to 20 carbon atoms and such that the carbon chain length from thealcohol includes at least four more carbon atoms than the largest alkylmoiety of the precursor dialkyl phosphite. In another embodiment, forexample, the transesterifying agent may be an ether of an alcohol withthe general formula, C_(n)H_((2n+1))O(CH₂CH₂O)_(m)H, where n is about 6to 16 and m is about 1 to 3. In one embodiment, the transesterifyingagent is an n-octanol.

The reaction of dialkyl phosphite and transesterifying agent can besummarized in Equations 2 and 3.

Varying the molar ratios in which the reactants are mixed can lead toproducts with varying composition and gelling activity. When a dialkyl(R¹, R¹) phosphite is reacted with an equimolar amount of a dissimilartransesterifying agent, such as alcohol (HOR²), a mixture of asymmetricdialkyl R¹, R² phosphite, symmetric dialkyl R², R² phosphite andunreacted symmetric dialkyl R¹, R¹ phosphite is obtained. The molarratio between the two products is dependent upon the relative ratio ofreactants used.

For example, if diethylphosphite is reacted with an equimolar amount ofa dissimilar alcohol, a mixture of asymmetric dialkylphosphite,symmetric dialkylphosphite and unreacted diethyl phosphite is obtained.As a more specific example, if diethylphosphite is reacted withn-octanol in a 1:1 molar ratio ethyloctylphosphite and dioctylphosphiteare obtained in a 2.2:1 molar ratio. On the other hand a 50% excessdiethylphosphite will increase the product ratio to 3.4:1 and a 100%excess diethylphosphite will give the products in a 4.7:1 ratio.

The transesterification reaction can be carried out in various ways. Inone method, for example, the reaction proceeds when reactants are mixedand the temperature is increased above ambient. For most reactants, wehave found that the transesterification reaction begins when thereactants are brought to about 100° C. and temperatures that decomposeor drive off the reactants should be avoided. Temperatures in the rangeof 100° C. to 200° C. are believed to be most useful. For example, thereaction of diethylphosphite and n-octanol proceeds at about 150 to 180°C. In another method, catalysis is used and may be carried out atgenerally ambient temperatures. For example, the transesterification ofdialkylphosphites can also be performed catalytically at roomtemperature in the presence alkali alkoxides, R²OM, where M=Li, Na, Kwith lithium alkoxides, R²OLi being a particularly useful catalyst. Thecatalyst loading required is as low as 1 mol %. Heating is not necessaryin such a catalyzed reaction.

After transesterifying, at least some transesterified asymmetric dialkylphosphite will be obtained along with the by products oftransesterification (i.e. R¹OH). The mixture generally will also containamounts of unreacted precursor dialkyl phosphite and unreactedtransesterifying agent.

The method proceeds by removing any unreacted transesterifying agent andany unreacted precursor dialkyl phosphite from the transesterifiedasymmetric dialkyl phosphite. The by products of transesterification mayalso be removed. The unwanted components can be removed in any one ofvarious ways. For example, distillation and chromatography are twopossible processes that may be used to remove the unwanted reactantsfrom the transesterified asymmetric dialkyl phosphite. For large scaleprocesses, where cost controls may be a factor, distillation may be mostuseful. Using distillation under reduced pressure, the transesterifyingagent and unreacted precursor dialkyl phosphite will distill off firstleaving a residue of transesterified asymmetric dialkyl phosphite. Anysymmetric dialkyl phosphite that is generated from thetransesterification reaction (i.e. dialkyl R², R² phosphite) willdistill after the asymmetric phosphite. As such, if distillation isstopped prior to the distillation of the asymmetric phosphite, theresidue will contain the transesterified asymmetric dialkyl phosphiteand the transesterified symmetric dialkyl phosphite. If desired,distillation can be continued to separately isolate these two species,although this will increase costs over leaving the residue with amixture of these species.

Following the foregoing procedures using diethylphosphite and n-octanol,ethyloctylphosphite (b.p. 110° C., 0.4 torr) can be obtainedsubstantially purely. Likewise, the foregoing procedures usingdiethylphosphite and n-decanol results in ethyldecylphosphite (b.p. 128°C., 0.3 torr). The ¹H and ³¹P NMR spectra of ethyloctylphosphite,(C₂H₅O)(C₈H₁₇O)P(O)H, are shown in FIGS. 2 and 3. A characteristicfeature which can be observed in the ¹H NMR spectrum of all phosphitesis the large P-H coupling constant of about 700 Hz.

After the desired phosphite products are separated from the unreactedprecursors, the transesterified asymmetric dialkyl phosphite is reactedto form an asymmetric dialkyl phosphate ester. The reaction mightgenerally be considered to be one of oxidation and may be carried out invarious ways using, for example, chlorine gas, hypochlorite, or otheroxidizing agents such as peroxide. The actual reactions and steps mayvary depending on the oxidizing agent used and, for example, may requireworking the product up as necessary to obtain the final dialkylphosphate ester.

For example, the reaction using chlorine gas produces a saltintermediate that requires hydrolysis to arrive at the final dialkylphosphate ester. The reaction with chlorine gas proceeds generallyaccording to Equation 4.

If hypochlorite is used for the conversion, the intermediates mayrequire acidification and neutralization to obtain the final dialkylphosphate ester.

These reactions may be exothermic. If temperature control is a factor,cooling or controlling the rate of addition of reactants may be useful.

The choice of the reactant used to convert the phosphite intermediatesinto final products may have an influence on the final productcomposition. For example, the use of chlorine gas tends to produce onlydiesters, whereas hypochlorite tends to react a dialkyl phosphite toproduce a dialkyl phosphate ester with some amount of monoalkylphosphate ester also being produced.

Since a product including both an asymmetric dialkyl phosphate ester andwith some amount of monoalkyl phosphate ester may be useful, as notedabove, a process where an amount of monoalkyl phosphate ester isproduced is not problematic. In fact, it may be desirable to addadditional monoalkyl phosphate ester to the finally obtained products toarrive at a desired composition. Such additional monoalkyl phosphateester may be from various sources and need not be generated from theabove-noted reaction, as desired.

As has been noted hereinbefore, it is desirable to obtain the asymmetricdialkyl phosphate ester substantially without, or at least with levelsbelow 1% by wt., of any phosphate triesters. The reaction tends not togenerate any triesters unless a trialkyl phosphite is present. As such,in one embodiment, the reactants, dialkyl phosphite and transesterifyingagent, may be selected to avoid problematic trialkyl phosphite content,which would lead to the eventual generation of triesters. Interestingly,however, it is noted that the step of removing the transesterifyingagent from the dialkyl phosphites, such as by distillation,chromatography, etc., may also remove problematic trialkyl phosphites.In particular, in a distillation process under reduced pressure, anytrialkyl phosphite is likely to distill off ahead of any transesterifieddialkyl phosphite. Other problematic volatile phosphorus compounds (i.e.those phosphorus impurities that distill without decomposition up to250° C.) that might be generated from the reaction, such as lowmolecular weight dialkyl phosphate esters (i.e. resulting from unreactedprecursor dialkyl phosphites) tend not to be present in the finalproduct, as their precursors may also be removed during the removalstep, for example, that including distillation.

An asymmetric dialkyl phosphate ester can be used as a liquidhydrocarbon gelling agent. The asymmetric dialkyl phosphate ester mayoptionally contain other dialkyl phosphate esters and/or monoalkylphosphate esters. In use, the asymmetric dialkyl phosphate ester may becombined with a crosslinking agent based on a polyvalent metal salt suchas, for example, any one of aluminum (III), iron (III) or iron (II), toform a polyvalent metal salt of the phosphate ester. The liquidhydrocarbons of interest are those useful in wellbore formation andtreatment operations. There are many such liquid hydrocarbons includingfor example, condensates, diesel oil, etc.

To seek to reduce and possibly avoid problems associated with equipmentfouling in refineries, phosphorus impurities that distill withoutdecomposition up to 250° C., and possibly up to 300° C., should besubstantially avoided in the gelled hydrocarbon or at least maintainedlow such that distillable phosphorus in the gelled hydrocarbondistillate is less than or equal to 10 ppm or possibly even less than orequal to 6 ppm. The use of asymmetric dialkyl phosphate ester of thepresent invention as gelling agents, can be useful in achieving such agoal.

A gelled liquid hydrocarbon can be prepared from a liquid hydrocarbonand a polyvalent metal salt of a phosphate ester, and may generallyinclude an amount of water. Optionally, the gelled liquid hydrocarbonalso may include any or all of a proppant material, a non emulsifier, adelayed gel breaker effective to break the gelled hydrocarbon fluid overa given period of time, or other components, as desired.

The gelled liquid hydrocarbon, having high viscosity and ability tosuspend solids, can be employed in wellbore processes, such as those foroilfield wellbore formation and treatment. For example, the gelledliquid hydrocarbon may be used in as a carrying medium for solids, suchas in wellbore stimulation processes such as fracturing or otherutilities where liquid hydrocarbons having a viscosity which is greaterthan their normal viscosity are useful.

In one embodiment, a gelled liquid hydrocarbon according to one or moreof the embodiments described above may be provided and introduced into awellbore to carry out a wellbore process. In one embodiment for example,there is provided a method for stimulating a wellbore including:providing a gelled liquid hydrocarbon and introducing the gelled liquidhydrocarbon to the wellbore. The gelled liquid hydrocarbon may be pumpedinto the wellbore and may be manipulated to treat the wellbore. In oneembodiment, the gelled liquid hydrocarbon may be manipulated, as bypressuring up, to stimulate the wellbore, as by fracturing a formationaccessed by the wellbore.

EXAMPLE 1

210.1 g (1.52 mol) of diethylphosphite was heated in the presence of 180g (1.38 mol) of n-octanol. The mixture was heated while stirring. At155° C., ethanol began to distil over. The reaction proceeded until 180°C. at which point no further evidence of ethanol production wasobserved. The resultant mixture was then distilled under vacuum of 0.4mbar. At 43-48° C., unreacted diethylphosphite was distilled withgreater than 99% purity. At 110° C., ethyloctylphosphite was distilledwith greater than 99% purity. The remaining residue was determined to bedioctylphosphite with greater than 99% purity. Purity was determined byexamination of the NMR spectrum of the materials. The ¹H NMR of theethyloctylphosphite produced is shown in FIG. 2, the ³¹P NMR is shown inFIG. 3.

The purified ethyloctylphosphite was converted toethyloctylchlorophosphate by reaction with chlorine and furtherhydrolyzed to ethyloctylphosphate. To chlorinate the phosphite, chlorinegas was bubbled through pure ethyloctylphosphite while stirring andcooling the solution with an ice water bath. The reaction proceeded tocompletion, and was indicated by a yellow discolouration of the solutiondue to the presence of dissolved unreacted chlorine. Subsequently thehydrochloric acid formed was removed under vacuum. Hydrolysis of theresulting chlorophosphates was accomplished by adding 124.8 g of a 25%sodium hydroxide at room temperature. The reaction is summarized inEquation 4.

The ¹H and ³¹P NMR spectra of ethyloctylphosphate thus obtained areshown in FIGS. 4 and 5. In the downfield portion of the proton spectrum,the doublet signal due to the P—H group (FIG. 4) has been replaced by asinglet signal due to the P—OH group.

The NMR spectra confirm that the materials produced are substantiallypure.

EXAMPLE 2

Gellation of diesel using the pure ethyloctylphosphate prepared inExample 1 was examined and compared with gellation using RhodafacLO-11A-LA. To gel the diesel, 1.2 mL of either Rhodafac LO-11A-LA orethyloctylphosphate was combined with 1 mL of Brine-Add OG-101C, acommercially available iron sulphate based crosslinker from

Brine-Add Fluids Ltd., in 200 mL of diesel while mixing at 1500 rpm,with an overhead mixer. The viscosity profiles of the gels obtainedusing the pure ethyloctylphosphate and Rhodafac LO-11A-LA weresignificantly different (FIG. 6). As can be noticed in FIG. 6 theviscosity of the gel obtained using the Rhodafac LO-11A-LA decreaseswith increasing the temperature. On the other hand the viscosity of thegel obtained using the pure ethyloctylphosphate from Example 1 increaseswith increasing the temperature.

EXAMPLE 3

Diethylphosphate was synthesized by adding diethylchlorophosphate (65 g;0.37 mol) to 30.14 g (0.75 mol) sodium hydroxide while stirring andcooling the solution with an ice water bath. The mixture was furtheracidified by the addition of 18.46 g (0.19 mol) sulphuric acid. The soobtained diethylphosphate was extracted with 100 mL diethyl ether andthe volatiles removed under vacuum leaving behind 57 g of product as acolourless liquid.

EXAMPLE 4

Dioctylphosphate was synthesized following a procedure similar to thatdescribed in Example 1 for the synthesis of ethyloctylphosphate. Thepurified dioctylphosphite, obtained as described in Example 1, wasconverted to dioctylchlorophosphate by reaction with chlorine andfurther hydrolyzed to dioctylphosphate with a 25% solution of sodiumhydroxide at room temperature.

EXAMPLE 5

Octylphosphate was synthesized by the hydrolysis ofoctyldichlorophosphate, which in turn is obtained in high yield as thereaction product of phosphorus oxychloride, POCl₃, and n-octanol. To 180g (1.17 mol) phosphorus oxychloride was added slowly under stirring152.4 g (1.17 mol) n-octanol. The mixture was stirred for 2 h at roomtemperature and the hydrochloric acid removed under vacuum. The soobtained product contains 95% octyldichlorophosphate along with 5%dioctylchlorophosphate. Further purification was achieved by distillingthe mixture at 98° C. and 0.4 mbar to give 145 g of pureoctyldichlorophosphate as a colourless liquid. Theoctyldichlorophosphate was hydrolyzed with 150 mL DI water at 50° C.Afterwards the water phase was removed, another 150 mL of DI water wereadded to the organic phase and the mixture further heated for 4 h at 90°C. Extraction of the product with 100 mL diethyl ether followed byremoval of the volatiles gave 121 g of octylphosphate as a viscouscolourless liquid.

EXAMPLE 6

Diethyloctylphosphate was prepared from octyldichlorophosphate andsodium ethoxide. To 114 g (0.46 mol) octyldichlorophosphate in 100 mLdiethyl ether 62.8 g (0.92 mol) sodium ethoxide were added in portionsunder ice cooling. The mixture was stirred for another hour at roomtemperature before being hydrolyzed with 200 mL of 1% solution of sodiumbicarbonate. The organic phase was recovered and the volatiles removedunder reduced pressure leaving behind 120 g of a pale yellow liquid.Further distillation at 129-132° C. and 0.2 mbar gave 81 g of pureproduct as a colourless liquid.

EXAMPLE 7

Gellation of diesel using diethylphosphate prepared in Example 3 wasexamined. Gelling of diesel was attempted by adding 1.2 mLdiethylphosphate and 1.0 mL of Brine-Add OG-101C, iron sulphate basedcrosslinker in 200 mL of diesel while mixing at 1500 rpm on an overheadmixer. No gellation of the diesel was noted.

EXAMPLE 8

Gellation of diesel using Dioctylphosphate prepared in Example 4 wasexamined. Gelling of diesel was attempted by adding 1.2 mLDioctylphosphate and 0.96 mL of Brine-Add OG-101C, an iron sulphatebased crosslinker in 200 mL of Diesel while mixing at 1500 rpm on anoverhead mixer. No gellation of the Diesel was noted.

EXAMPLE 9

Gellation of diesel using octylphosphate prepared in Example 4 wasexamined. Gelling of diesel was attempted by adding 1.2 mLoctylphosphate and 0.96 mL of Brine-Add OG-101C, iron sulphate basedcrosslinker in 200 mL of diesel while mixing at 1500 rpm on an overheadmixer. No gellation of the diesel was noted.

EXAMPLE 10

Diethylphosphate prepared in Example 3 was mixed withethyloctylphosphate prepared in Example 1 at a ratio of 1:3.5 by weight.

EXAMPLE 11

Dioctylphosphate prepared in Example 4 was mixed withethyloctylphosphate prepared in Example 1 at ratios of 1:5 by weight.

EXAMPLE 12

Diethyloctylphosphate prepared in Example 6 was mixed withethyloctylphosphate prepared in Example 1 at ratios of 1:9 by weight.

EXAMPLE 13

Octylphosphate prepared in Example 3 was mixed with ethyloctylphosphateprepared in Example 1 at a ratio of 1:19 and 1:5.66 by weight.

EXAMPLE 14

Gellation of diesel using the mixture prepared in Example 10 wasexamined. To gel the diesel, 1.2 mL of the mixture from Example 10 wascombined with 1.00 mL of Brine-Add OG-101C, an iron sulphate basedcrosslinker in 200 mL of diesel while mixing at 1500 rpm on an overheadmixer. The viscosity profiles of the gels subsequently obtained areshown in FIG. 7. Addition of diethylphosphate to ethyloctylphosphateslightly decreases the overall viscosities of gels as compared with gelsprepared from pure ethyloctylphosphate.

EXAMPLE 15

Gellation of diesel using the mixture prepared in Example 11 wasexamined. To gel the diesel, 1.2 mL of the mixture from Example 11 wascombined with 1.00 mL of Brine-Add OG-101C, an iron sulphate basedcrosslinker in 200 mL of diesel while mixing at 1500 rpm on a Caframo®overhead mixer. The viscosity profiles of the gels subsequently obtainedare shown in FIG. 8. As was observed with the symmetric diethylphosphatein Example 14, the longer alkyl chain, dioctylphosphate, decreases theoverall viscosity of the gels as compared with gels prepared from pureethyloctylphosphate. An increase in the time required for the gel toform was also observed.

EXAMPLE 16

Gellation of diesel using the mixture prepared in Example 12 wasexamined. To gel the diesel, 1.2 mL of the mixture from Example 12 wascombined with 1.00 mL of Brine-Add OG-101C, an iron sulphate basedcrosslinker in 200 mL of diesel while mixing at 1500 rpm on an overheadmixer. The viscosity profiles of the gels subsequently obtained areshown in FIG. 9. Adding 10% diethyloctylphosphate to pureethyloctylphosphate decreases the overall viscosity of the gels ascompared with gels prepared from pure ethyloctylphosphate (FIG. 9).

EXAMPLE 17

Gellation of diesel using the mixtures prepared in Example 13 wasexamined. To gel the

Diesel, 1.2 mL of the respective mixture from Example 13 was combinedwith 1.0 mL of Brine-Add OG-101C, an iron sulphate based crosslinker in200 mL of diesel while mixing at 1500 rpm on an overhead mixer. Theviscosity profiles of the gels subsequently obtained are shown in FIG.10.

Surprisingly, adding 5% octylphosphate to ethyloctylphosphate increasesthe viscosity of the gel as compared with the corresponding gel preparedfrom ethyloctylphosphate up to a temperature of 70° C. (FIG. 10). Adecrease in the time required for the diesel to gel was noticed with thematerial containing 5% octylphosphate with 95% ethyloctylphosphate.Adding 15% of the octylphosphate has a detrimental effect as theviscosity of the gel is decreased over the entire temperature range. Itappeared that concentrations beyond about 10% by weight ofoctylphosphate in the ethyloctylphosphate failed to provide thebeneficial gelling results.

EXAMPLE 18

210.1 g (1.52 mol) diethylphosphite was heated in the presence of 180 g(1.38 mol) n-octanol. The mixture was heated while stirring. At 150° C.,ethanol began to distil over. The reaction proceeded until 180° C. atwhich point no further evidence of ethanol production was observed. Theresultant mixture was then distilled under vacuum of 0.6 mbar. At 43-48°C., unreacted diethylphosphite was distilled, leaving behind 252 g of a1:2.2 mole mixture of ethyloctylphosphite and dioctylphosphite. Themixture of phosphites was converted to the correspondingchlorophosphates by reaction with chlorine and further hydrolyzed toethyloctylphosphate and dioctylphosphate respectively. To chlorinate thephosphites, chlorine gas was bubbled through the solution of phosphiteswhile stirring and cooling the solution with an ice water bath. Thereaction proceeded to completion, and was indicated by the solutionturning yellow due to the presence of dissolved unreacted chlorine.Afterwards the hydrochloric acid formed was removed under vacuum.Hydrolysis of the resulting chlorophosphates was accomplished by adding166 g of a 25% sodium hydroxide at room temperature. 256 g of acolourless liquid consisting of ethyloctylphosphate 69% anddioctylphosphate 31% was obtained.

EXAMPLE 19

252 g of a 1:2.2 mixture of ethyloctylphosphite and dioctylphosphite wasprepared as described in Example 20. The phosphites were furtherconverted to the corresponding phosphates by reacting them with sodiumhypochlorite. The ethyloctylphosphite and dioctylphosphite mixture wasadded to 918 g sodium hypochlorite solution (10-13% as active chlorine).The addition was performed dropwise with water cooling (temperaturebelow 30° C.) and the pH maintained between 10-11.5 with 25% NaOH. Afterall the phosphite was added the pH is raised to about 11-11.5 and thereaction mixture was stirred until the pH stayed above 11. Afterwardsthe mixture was treated dropwise with sulfuric acid (98%) until a pH of2 was measured. Two phases formed and the upper yellow organic phase wasseparated. 265 g of a viscous colourless liquid was obtained, thecomposition confirmed with ³¹P NMR, consisting of 3.6% octylphosphate(83.5 ppm, singlet), 66.3% ethyloctylphosphate (δ 1.87 ppm, singlet) and30.1% dioctylphosphate (82.1 ppm, singlet).

EXAMPLE 20

50 g (0.38 mol) of n-octanol was premixed with 3.5 mL of a 2.2M methanolsolution of lithium methoxide. The mixture was rapidly added at roomtemperature to 53 g (0.38 mol) diethylphosphite. The colour of thesolution changed from colourless to yellow. The resultant mixture wasthen distilled under vacuum of 1.00 mBar. At 23° C. ethanol was removed.At 43-48° C., unreacted diethyl phosphite was distilled, with theremaining 65 g comprising a 1.6:1 mole mixture of ethyloctylphosphiteand dioctylphosphite. The phosphites were further converted to thecorresponding phosphates by reacting them with sodium hypochlorite asdescribed in Example 19.

EXAMPLE 21

20 g (0.15 mol) of n-octanol was premixed with 0.29 g sodiumtert-butoxide and 0.46 mL tetramethylethylenediamine. The mixture wasadded rapidly at room temperature to 21.23 g (0.15 mol)diethylphosphite. The resultant mixture was then distilled under vacuumof 1.00 mBar. At 23° C. ethanol was removed. At 43-48° C., unreacteddiethylphosphite was distilled, with the remaining 65 g comprising a1.6:1 mole mixture of ethyloctylphosphite and dioctylphosphite. Thephosphites are further converted to the corresponding phosphates byreacting them with sodium hypochlorite as described in Example 19.

EXAMPLE 22

50 g (0.38 mol) of n-octanol was premixed with 3.5 mL of a 2.2M Methanolsolution of lithium methoxide. The mixture was rapidly added at roomtemperature to 53 g (0.38 mol) diethylphosphite and the colour of thesolution was observed to change from colourless to yellow. The ethanolformed during the reaction was removed under vacuum, leaving behind 87.3g of a mixture of diethylphosphite (13 mole %), ethyloctylphosphite (52mole %), and dioctylphosphite (35 mole %). The phosphites were furtherconverted to the corresponding phosphates by reacting them with sodiumhypochlorite. This was achieved by adding the mixture of phosphites to343 g sodium hypochlorite solution (10-13% as active chlorine). Theaddition was performed dropwise with water cooling (temperature below30° C.) and the pH maintained between 10-11.5 with 25% NaOH. After allthe phosphite was added the pH was raised to about 11-11.5 and thereaction mixture was stirred until the pH remained above 11.Subsequently the mixture was treated dropwise with hydrochloric aciduntil a pH of 2 was measured. 10 g of sodium chloride was also added inorder to aid in extraction of diethylphosphate from the water phase. Twophases were observed to form and the upper yellow organic phase wasseparated. 94 g of a viscous colourless liquid are obtained and thecomposition determined by ³¹P NMR. The composition consisted of 3.6 mol% octylphosphate (δ 3.5 ppm, singlet), 52.7 mol % ethyloctylphosphate (δ1.8 ppm, singlet), 17 mol % diethylphosphate (δ 1.6 ppm, singlet), and26.7 mol % dioctylphosphate (δ 82.1 ppm, singlet).

EXAMPLE 23

50 g of a 3:7 weight ratio of n-hexadecanol:n-octadecanol was premixedwith 1.7 mL of a 2.2M Methanol solution of lithium methoxide. Themixture was rapidly added at room temperature to 26.4 g (0.19 mol)diethylphosphite. The resultant mixture was then distilled under vacuumof 1.00 mBar. At 23° C. ethanol was removed. At 43-48° C., unreacteddiethylphosphite was distilled in greater than 99% purity, leavingbehind 61.5 g of a mixture of ethylhexadecylphosphite,ethyloctadecylphosphite, dihexadecylphosphite, dioctadecylphosphite andhexadecyloctadecylphosphite as a white solid. The resulting phosphiteswere subsequently converted to the corresponding phosphates by reactingthem with sodium hypochlorite. To accomplish this, 50 g of the abovemixture was added to 104 g sodium hypochlorite solution (10-13% asactive chlorine). The addition was performed in portions at temperaturesbetween 35-40° C. while maintaining the pH between 9-11.5 with 25% NaOH.After all the phosphite was added the pH was raised to about 11-11.5 andthe reaction mixture was stirred at 60° C. until the pH remained above11. Afterwards the mixture was treated dropwise with 98% sulphuric aciduntil a pH of 2 was measured. The solid product was separated from thewater phase by filtration. 51.3 g of a white solid was obtainedconsisting of ethylhexadecylphosphite, ethyloctadecylphosphate,dihexadecylphosphate, dioctadecylphosphate andhexadecyloctadecylphospate. The ratio between the asymmetric andsymmetric phosphates was determined to be 1.9:1 as measured by ³¹P NMR.

EXAMPLE 24

40 g (0.2 mol) of dibutylphosphite was heated while stirring in thepresence of 26.8 g (0.2 mol) of n-octanol. At 180° C., butyl alcohol wasobserved to distil. The reaction proceeded until 200° C. at which pointno further evidence of butyl alcohol production was observed. Theresultant mixture was then distilled under vacuum of 1.00 mBar. At 92°C., unreacted dibutylphosphite was distilled in greater than 99% purity,leaving behind 42 g of a mixture of butyloctylphosphite anddioctylphosphite. The resulting phosphites were further converted to thecorresponding phosphates by reacting them with sodium hypochlorite. Toachieve this, 42 g of the mixture of dioctylphosphite andbutyloctylphosphite was added to 148 g sodium hypochlorite solution(10-13% as active chlorine). The addition was done dropwise with watercooling while maintaining a temperature between 30-40° C. andmaintaining the pH between 9-11.5 using 25% sodium hydroxide. Afteraddition of the phosphite, the pH was raised to about 11-11.5 and thereaction mixture was stirred until the pH remained above 11. Afterwardsthe mixture is treated dropwise with sulphuric acid (98%) until a pH of2 was observed. At this point two phases formed and the upper organicphase was separated. 48 g of a viscous colourless liquid was obtainedconsisting of butyloctylphosphite and dioctylphosphate. In the ³¹P NMRthe two dialkylphosphates are isochronous and cannot be distinguished.Hence only one singlet at 2.1 ppm was observed.

EXAMPLE 25

60 g (0.43 mol) diethylphosphite was heated in the presence of 60 g(0.43 mol) ethylene glycol phenyl ether. The mixture was heated whilestirring. At 150° C., ethanol was observed to distil. The reactionproceeded until 180° C. at which point no further evidence of ethanolproduction was observed. The resultant mixture was then distilled undervacuum of 0.6 mbar. At 43-48° C., unreacted diethylphosphite wasdistilled leaving 252 g of a 1:2.2 mol mixture ofethyl(ethyleneglycolphenlylether)phosphite anddi(ethyleneglycolphenlylether)phosphite. The resulting phosphites werefurther converted to the corresponding phosphates by reacting them withsodium hypochlorite as described in Example 19.

EXAMPLE 26

The reaction of 54 g (0.39 mol) diethylphosphite with 67.5 g (0.35 mol)diethyleneglycol monohexylether, followed by the oxidation of theresulting phosphites with sodium hypochlorite was carried out as inExample 25.

EXAMPLE 27

Gellation of Diesel using the mixtures prepared in Example 18-26 wasexamined. To gel the Diesel, 1.2 mL of the respective mixture fromExample 18-26 was combined with 1.0 mL of Brine-Add OG-101C, an ironsulphate based crosslinker in 200 mL of diesel while mixing at 1500 rpmon an overhead mixer. With the exception of the product from Example 25all other materials gelled the diesel. The viscosity profiles of thegels subsequently obtained are shown in FIGS. 11 and 12.

Comparative viscosity measurements in the temperature range 90-130° C.indicate that at high temperatures the gels prepared with products fromExample 1, 18, and 20 produce higher viscosity gels than the gelsprepared using the commercial product Rhodafac LO-11A-LA (FIG. 13). Thegeneral trend indicates that increasing the asymmetricethyloctylphosphate content of the phosphate ester mixture used togenerate oil based gels results in an increase in the thermal stabilityof the gels.

Conversely, the presence of small amounts of monoester in the phosphateester mixture will increase the low temperature viscosities of the gelsand decrease the high temperature stability of the gels.

EXAMPLE 28

Hydrocarbon gels were prepared using either Rhodafac LO-11A-LA, or oneof the phosphate esters produced in Examples 1, 3, 6, 18 or 20. Thephosphate esters were added on to a typical fracturing fluidhydrocarbon, C-2000. C-2000 is a commercially available mixture ofhydrocarbons which has an initial boiling point of about 110° C. and is80% distilled at about 250° C. Each phosphate ester sample was addedsuch that the C-2000 contained 950 ppm total phosphorus. A gel was thenachieved by adding 2 mL of Brine-Add OG-101C, an iron sulphate basedcrosslinker, 0.8mL of Brine-Add OG-103B, a slurry of magnesium oxide inmineral oil breaker to 400mL C2000 while mixing at 1500 rpm on anoverhead mixer. Samples were then placed in 500 mL cylindrical stainlesssteel cells and rolled at 80° C. for 16h to simulate a typicalfracturing treatment. 100 mL of the resulting fluid was distilledaccording to ASTM method D86-04b, “Standard Method for Distillation ofPetroleum Products at Atmospheric Pressure”. The distillation wasperformed in triplicate for each system and samples collected submittedto a contract laboratory for Phosphorus Analysis by an ICP Analyzer(Inductively Coupled Plasma). The reported detection limit for themethod is 0.2 ppm phosphorus. The average phosphorus content in each ofbase fluids is summarized in Table 1.

TABLE 1 Distillable Phosphorus From Various Phosphate Esters VolatilePhosphorus Gellant (ppm) Rhodafac LO-11A-LA  32.2 Ethyloctylphosphate(Product Example 1)  0.7 Diethylphosphate (Product Example 3)  13.9Diethyloctylphosphate (Product Example 6) 207.5 MixedEthyloctylphosphate:Dioctylphosphate  2.9 (Product Example 18) MixedEthyloctylphosphate:Dioctylphosphate:Octylphosphate  5.7 (ProductExample 20)

As can be seen in Table 1 the highest phosphorus values 207.5 and 13.9respectively, were found in the samples containing the triester;diethyloctylphosphate and the diester, diethylphosphate. The samplescontaining the asymmetric diester, ethyloctyiphosphate, the diester,dioctylphosphate, and the monoester, octylphosphate have the leastcontribution to distillable phosphorus. The distillable phosphorusvalues for these samples are in the range 0.7 ppm to 5.7 ppm. The gelprepared from ethyloctyiphosphate contained virtually no distillablephosphorus indicating that this compound does not contribute todistillable phosphorus.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to those embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein, but is to beaccorded the full scope consistent with the claims, wherein reference toan element in the singular, such as by use of the article “a” or “an” isnot intended to mean “one and only one” unless specifically so stated,but rather “one or more”. All structural and functional equivalents tothe elements of the various embodiments described throughout thedisclosure that are know or later come to be known to those of ordinaryskill in the art are intended to be encompassed by the elements of theclaims. Moreover, nothing disclosed herein is intended to be dedicatedto the public regardless of whether such disclosure is explicitlyrecited in the claims. No claim element is to be construed under theprovisions of 35 USC 112, sixth paragraph, unless the element isexpressly recited using the phrase “means for” or “step for”.

1. A method for producing an asymmetric dialkyl phosphate ester, themethod comprising: (a) reacting a precursor dialkyl phosphite with atransesterifying agent to obtain a transesterified asymmetric dialkylphosphite; (b) removing any unreacted transesterifying agent and anyunreacted precursor dialkyl phosphite from the transesterifiedasymmetric dialkyl phosphite; and (c) reacting the transesterifiedasymmetric dialkyl phosphite to form an asymmetric dialkyl phosphateester.
 2. The method of claim 1 wherein the precursor dialkyl phosphiteis symmetric.
 3. The method of claim 1 wherein the precursor dialkylphosphite includes a straight chain C1 to C4 alkyl group.
 4. The methodof claim 1 wherein the precursor dialkyl phosphite is selected from thegroup consisting of dimethyl phosphite, diethyl phosphite, dipropylphosphite and dibutyl phosphite.
 5. The method of claim 1 wherein theprecursor dialkyl phosphite is diethyl phosphite.
 6. The method of claim1 wherein the transesterifying agent includes a carbon chain backbonethat is dissimilar to the alkyl groups of the precursor dialkylphosphite.
 7. The method of claim 1 wherein the transesterifying agentis a primary alcohol with a carbon chain length including 6 to 20 atomsand such that the carbon chain length from the primary alcohol includesat least four more carbon atoms than the largest alkyl group of theprecursor dialkyl phosphite.
 8. The method of claim 1 wherein thetransesterifying agent is an ether of an alcohol with a general formula,C_(n)H_((2n+1))O(CH₂CH₂O)_(m)H, where n is about 6 to 16 and m is about1 to 3 and such that the ether of an alcohol includes a backbone chainlength including at least four more atoms than the largest alkyl groupof the precursor dialkyl phosphite.
 9. The method of claim 1 wherein thetransesterifying agent is an n-octanol.
 10. The method of claim 1wherein reacting a precursor dialkyl phosphite with a transesterifyingagent also generates a transesterified symmetric dialkyl phosphite. 11.The method of claim 1 wherein removing uses distillation.
 12. The methodof claim 1 wherein reacting a precursor dialkyl phosphite with atransesterifying agent also generates a transesterified symmetricdialkyl phosphite and removing produces a mixture of the transesterifiedasymmetric dialkyl phosphite and the transesterified symmetric dialkylphosphite.
 13. The method of claim 1 wherein reacting thetransesterified asymmetric dialkyl phosphite to form an asymmetricdialkyl phosphate ester includes oxidation of the transesterifiedasymmetric dialkyl phosphite.
 14. The method of claim 1 furthercomprising adding an amount of monoalkyl phosphate ester to theasymmetric dialkyl phosphate ester.
 15. An asymmetric dialkyl phosphateester according to the formula:

R¹ is a straight chained alkyl group having 1 to 4 carbon atoms; and R²is alkyl group having 6 to 20 carbon atoms or an alkoxyalkyl group,C_(n)H_((2n+1))O(CH₂CH₂O)_(x)CH₂CH₂—, where n is about 6 to 16 and x isabout 1 or 2, and wherein any amount of phosphate triester that distilswithout decomposition at temperatures up to 250° C. (ASTM D86) ismaintained below 1% by weight.
 16. The asymmetric dialkyl phosphateester of claim 15 wherein R² is a C8 alkyl group.
 17. The asymmetricdialkyl phosphate ester of claim 15 wherein R¹ is an ethyl group. 18.The asymmetric dialkyl phosphate ester of claim 15 wherein any amountphosphorus impurities that distil without decomposition at temperaturesup to 250° C. (ASTM D86) is maintained below 1% by weight.
 19. Theasymmetric dialkyl phosphate ester of claim 15 wherein R² has a chainlength of about 4 atoms more than the chain length of R¹.